DESCRIPTION

These utility routines provide management of pools of fixed-sized areas
of memory. Resource pools set aside an amount of memory for exclusive use
by the resource pool owner. This can be used by applications to guarantee
the availability of a minimum amount of memory needed to continue opera-
tion independent of the memory resources currently available from the
system-wide memory allocator (malloc(9)). The pool manager obtains memory
by using the special-purpose memory allocator palloc() passed to
pool_init(), for extra pool items in case the number of allocations
exceeds the nominal number of pool items managed by a pool resource. This
temporary memory will be automatically returned to the system at a later
time.

CREATING A POOL

The function pool_init() initializes a resource pool. The arguments are:
pool Specifies the pool storage to be initialized.
size Specifies the size of the memory items managed by the
pool.
align Specifies the memory address alignment of the items
returned by pool_get(). This argument must be a power
of two. If zero, the alignment defaults to an
architecture-specific natural alignment.
align_offset The offset within an item to which the align parame-
ter applies.
flags Specifies various flags set on the pool at creation
time.
wmesg The message passed on to tsleep(9) if pool_get() must
wait for items to be returned to the pool.
palloc The back-end allocator used to manage the memory for
the pool. palloc() may be NULL, in which case the
pool manager uses the pool_allocator_kmem allocator
which uses uvm_km_kmemalloc(9) and uvm_km_free(9) to
allocate and release memory using the kmem_map
(see uvm(9)). It is recommended to set this to
pool_allocator_nointr if the pool will never be ac-
cessed in interrupt context, since that allocator is
much more efficient.

DESTROYING A POOL

The pool_destroy() function destroys a resource pool. It takes a single
argument pp identifying the pool resource instance.

SETTING DRAIN CALLBACK

The pool_set_drain_hook() function can be used to specify a function that
will be called when memory is running low. The callback fun will be
called with the arguments arg which is the third argument to
pool_set_drain_hook() and flags which will have PR_WAITOK set if the
drain hook is allowed to sleep.

ALLOCATING ITEMS FROM A POOL

pool_get() allocates an item from the pool and returns a pointer to it.
pp The handle identifying the pool resource instance.
flags One or more of PR_URGENT, PR_WAITOK or PR_LIMITFAIL, that
define behaviour in case the pooled resources are depleted.
If no resources are available and PR_WAITOK is given, this
function will wait until items are returned to the pool.
Otherwise pool_get() returns NULL. If PR_URGENT is specified
and no items are available and palloc() cannot allocate a
new page, the system will panic (XXX). If both PR_LIMITFAIL
and PR_WAITOK are specified, and the pool has reached its
hard limit, pool_get() will return NULL without waiting, al-
lowing the caller to do its own garbage collection; however,
it will still wait if the pool is not yet at its hard limit.

RETURNING ITEMS TO A POOL

pool_put() returns the pool item pointed at by item to the resource pool
identified by the pool handle pp. If the number of available items in the
pool exceeds the maximum pool size set by pool_sethiwat() and there are
no outstanding requests for pool items, the excess items will be returned
to the system by calling prelease().
pp The handle identifying the pool resource instance.
item A pointer to a pool item previously obtained by pool_get().

PRIMING A POOL

pool_prime() adds items to the pool. Storage space for the items is allo-
cated by using the page allocation routine specified to pool_init().
pool_prime()
pp The handle identifying the pool resource instance.
nitems The number of items to add to the pool.
This function may return ENOMEM in case the requested number of items
could not be allocated. Otherwise, the return value is 0.

SETTING POOL RESOURCE WATERMARKS

A pool will attempt to increase its resource usage to keep up with the
demand for its items. Conversely, it will return unused memory to the
system should the number of accumulated unused items in the pool exceed a
programmable limit. The limits for the minimum and maximum number of
items which a pool should keep at hand are known as the high and low
watermarks. The functions pool_sethiwat() and pool_setlowat() set a
pool's high and low watermarks, respectively.
pool_sethiwat()
pp The handle identifying the pool resource instance.
n The maximum number of items to keep in the pool. As items
are returned and the total number of pages in the pool is
larger than the maximum set by this function, any completely
unused pages are released immediately (by calling
prelease()). If this function is not used to specify a max-
imum number of items, the pages will remain associated with
the pool until the system runs low on memory, at which point
the VM system will try to reclaim unused pages.
pool_setlowat()
pp The handle identifying the pool resource instance.
n The minimum number of items to keep in the pool. The number
of pages in the pool will not decrease below the required
value to accommodate the minimum number of items specified
by this function. Unlike pool_prime(), this function does
not allocate the necessary memory up-front.

SETTING HARD LIMITS

The function pool_sethardlimit() sets a hard limit on the pool to n
items. If the hard limit is reached warnmess will be printed to the con-
sole, but no more than every ratecap seconds. Upon successful completion,
a value of 0 is returned. The value EINVAL is returned when the current
size of the pool already exceeds the requested hard limit.

POTENTIAL PITFALLS

Note that undefined behaviour results when mixing the storage providing
methods supported by the pool resource routines.
The pool resource code uses a per-pool lock to protect its internal
state. If any pool functions are called in an interrupt context, the
caller must block all interrupts that might cause the code to be reen-
tered.

POOL CACHES

Another set of functions are available as extensions to the pool manager.
The pool cache functions automatically call constructors and destructors
when objects are allocated from the pool or returned to it. They have
similar semantics as the other pool functions. The pp argument to
pool_cache_init() must already be initialized.
Objects are not immediately deconstructed when put into the pool cache.
Instead, they are maintained for future allocations. When the system
determines that memory needs to be reclaimed, then the deconstructor is
called on each free object and it is placed back into the pool. The ctor
and dtor functions are passed arg and a pointer to the object, in that
order. The ctor is also passed the same flags that are passed to
pool_cache_get(). The pool_cache_destruct_object() function deconstructs
and puts an object back into the pool immediately.
pool_cache_invalidate() deconstructs all cached objects and releases
their memory.
Pool caches are also commonly referred to as a slab allocator.

DIAGNOSTICS

Pool usage logs can be enabled by defining the compile-time option
POOL_DIAGNOSTIC.

DEBUGGING

To debug a misbehaving pool, a kernel can be compiled with the
MALLOC_DEBUG option and memory debugging on pools can be enabled with the
PR_DEBUG flag passed in the flags argument in the call to pool_init().
See malloc(9) for more information about MALLOC_DEBUG.